Constructive meditation practices are those that strengthen psychological patterns (hopefully patterns associated with well-being) — for example, a meditator might hold a loved one in mind while repeating phrases like “may you be well” and attempting to generate and amplify the positive intention.
Deconstructive meditation practices are those that analyze experience into component parts, which are often then analyzed and decomposed again. In vipassana mediation, for example, the meditator might observe a pain in the leg, and immediately move past seeing it as one big chunky thing and instead see it as the rich dynamic tapestry of arising and changing and passing away sensations that it is.
This distinction is articulated in a cognitive science research paper by Richard Davidson and others.
Holding this distinction, we might ask whether mindfulness is constructive or deconstructive or both or neither.